You never need to upload the documents in the Netherlands, their software doesn't have such an option. But technically you're expected to keep the documents at

You never need to upload the documents in the Netherlands, their software doesn't have such an option. But technically you're expected to keep the documents at least until you receive the "definitieve aanslag" and if you're nitpicking I think there is a 7 year term for the tax services to come back on your filed taxes and change things or demand proof. Practically that doesn't happen if you accepted their pre-filled numbers and they match your employers. But if you're a freelancer or other non-standard case I would keep digital copies for a few years just to be sure.